Air induction systems for automotive applications of internal combustion engines should attenuate induction noise and filter the incoming air with minimal system restriction. Internal combustion engines, particularly those having four cylinders, may create significant low frequency induction noise. The noise, generated at the valves and inside the combustion chambers, is transmitted through the intake manifold, throttle body, air inlet ducting, air cleaner and the air inlet snorkel. The high energy noise, combined with air pulsations residing in the induction system, may impart vibration to the induction system components resulting in surface radiated noise. Such driving energy, which generates such surface noise, is typically in the range of 20 to 420 Hz.
When the driving energy frequency closely matches a natural frequency of an induction system component surface, the surface may resonate, emitting undesirable resonance noise. To reduce or eliminate noise generated by surface resonance, induction component structures should be sufficiently stiffened to ensure that the components first natural frequency is above the range of the driving energy generated in the induction system. The first natural frequency target must be met for underhood temperature conditions. For plastic components, stiffness drops at high temperature due to a shift in the modulus of elasticity of the plastic with increasing temperature.
A typical method for achieving high stiffness to increase natural frequency values is to locate stiffening ribs perpendicular to the surface being addressed. The stiffening ribs increase the area moment of inertia in the direction of vibration. Automotive design constraints, however, limit the usefulness of the ribs as a solution to such vibration. Typical stiffening ribs, located on the inside of the component surface, are height limited. Beyond height limits defined by the material, "sink" marks in the component exterior surface appear. Such surface imperfections have an unacceptable appearance quality. In addition, placement of ribs on the exterior of the component surface is typically not an option due to packaging limitations requiring maximization of component internal volumes for the purpose of acoustical noise attenuation. Structural ribs located on outside part surfaces force the component walls to be shifted inwardly, assuming fixed component exterior dimensions, thereby decreasing internal volumes.
Exotic materials and complex component surface configurations are also options to increase part surface stiffness. These solutions are unacceptable in that material and tooling costs are unacceptably high.